This guide was started by me but is being added to by members of various forums. If you have an addition for this guide send me a PM and I will make any necessary changes or additions. Any input to the guide will be credited.

Following this guide are others. Browse through them all and choose the one that suits you. There is also a wealth of links in Ed's guide in the next post so don't stop here!

Introduction

This guide is intended to help give aeromodelers enough simplified knowledge to choose from the bewildering array of components available, and to assemble appropriate power systems for their model aeroplanes. This guide is not intended as an answer to everyone’s questions, but as a basic introduction so that we have a reference to help understand this subject.

Using this Guide

Choosing a power system is a more complicated procedure than I first thought when I started putting this guide together. Often choosing a power system includes a fair bit of educated guesswork. Although it is possible to accurately calculate every important bit of data to determine the optimum power system for a model, most of us will prefer the “educated guess” approach. Unfortunately there is some knowledge required to make an “educated guess”, and that is hopefully what this guide will provide.

For those completely new to the world of aeromodeling who also intend building a new model, I suggest starting from the beginning of this guide. Even if it doesn’t all make sense at first, it will eventually. Some trial and error might not seem like the most economical way of learning but it can sometimes be the most effective.

Many people will only want to access certain information, so each topic has a heading. Scroll down to see if you can find the information you need.

Wing Loading and Stall Speed

Wing loading is the loaded weight of an aircraft divided by the area of the wing. It is broadly reflective of the aircraft's lift-to-mass ratio, which affects its rate of climb, load-carrying ability, and turn performance.

Many aeromodelers try their hardest to make models as light as possible. This is because a model with a light wing loading is easier to fly as it has a slower stall speed. In a banked turn an aeroplane is subjected to a gravitational force (G) which increases its weight, the same as a weight on the end of some string gets heavier if you spin it around like a lasso. The heavier the wing loading in a banked turn, the higher the stall speed gets. Other factors that can affect the stall speed of a model aeroplane are aerofoil shape and aspect ratio. The only way of giving a heavy model a light wing loading is to increase the size of the wings. So a model with a high wing loading will have a high stall speed, which will become even higher in high G manoeuvres. Because we land our models at or slightly above their stall speed, a model with a high wing loading will have a high landing speed which can call for some very good piloting skills.

The most important terms you need to understand when choosing components for your electric model are Volts, Amps and Watts.Here is the "Hydraulic analogy" from Wikipedia which explains these terms in a simple way.

"The hydraulic analogy is sometimes used to explain electric circuits by comparing them to water-filled pipes, voltage is likened to water pressure - it determines how fast the electrons will travel through the circuit. Current (in amperes), in the same analogy, is a measure of the volume of water that flows past a given point, the rate of which is determined by the voltage, and the total output measured in watts. The equation that brings all three components together is: volts × amperes = watts".

As we will see, the size and efficiency of a motor and the load imposed on it by the propeller affects the Volts and Amps. The idea is to choose a motor, battery, esc and propeller combination that will fly your model in a desired manner within the specifications of the components, preferably at around the peak efficiency of the motor. This will be covered in more detail in the following sections.

Choosing a Motor

Weight Power and Dimensions

The most important thing to keep in mind before choosing a motor is its weight and dimensions. We would all agree that extra weight added to a model to achieve the correct centre of gravity is undesirable. I personally would prefer to have a larger, heavier and more powerful motor than a smaller less powerful motor and a lump of lead in my models. Sometimes there is no choice but to use lead, but just don’t forget about the relationship between the weight of your motor and the centre of gravity of your model. The dimensions of a motor are obviously important. Don’t buy it if it won’t fit in your model.

You will want a level of performance suitable for the type of model you are powering. A 3D model will need thrust greater than 1:1, and a scale WW1 biplane will need considerably less. Here is a table giving performance in Watts per pound. Remember that if you are running your motor above its maximum efficiency the Watts per pound rule won’t be accurate, as a higher percentage of the Watts going into the motor will be producing heat instead of power.

70-90 watts/lb. Trainer and slow flying aerobatic models.

90-110 watts/lb. Sport aerobatic and fast flying scale models.

110-130 watts/lb. advanced aerobatic and high speed models

130-150 watts/lb. Lightly loaded 3D models and ducted fans.

150-200+ watts/lb. Unlimited performance 3D models.

Motors

Inrunner or outrunner?

Now you have an idea of the weight and power you will need for your model, what sort of motor is best, an inrunner or outrunner?

Both have their pros and cons.

Inrunners

Inrunners are constructed with the magnets attached directly to the shaft, which is surrounded by the copper windings. Because the magnets are close to the shaft it spins very quickly. This means they produce high rpm but low torque. This high rpm can be converted into torque by using a gearbox (see the section below on gearboxes).

Inrunners are more efficient and powerful, but need a gearbox to drive large propellers. They produce high revs per volt (Kv) compared to outrunners. For models requiring a small prop running at high speed like a Zagi (wing), pylon racers and ducted fans, inrunners without gearboxes are popular. Once a gearbox is used there are even more pros and cons. Gearboxes are an extra expense, require maintenance and can be noisy, but you will still get the best efficiency and power with a geared inrunner spinning a large prop. This is the reason why all competitive F5b models still use geared inrunners.

Outrunners

Outrunners are constructed with the copper windings on the inside. The shaft is attached to a “bell”, or casing that contains the magnets, which spin around the copper windings. Because the extra weight of the bell and magnets are further out from the shaft it acts like a flywheel. Generally outrunners produce lower RPM at higher torque than inrunners due to the way they are made. This enables an outrunner to spin a larger prop without a gearbox.

This means no maintenance, quieter operation and cheaper purchase price (no gearbox). These factors outweigh the higher efficiency and power of the inrunner for most sport flyer's.

kv

kv is simply the revolutions per minute (rpm) an electric motor will spin at per per volt when under no load. You could think of high and low kv like the difference between a high performance 2 stroke racing motorcycle engine compared to a 4 stroke Harley Davidson motorcycle engine. Just say they put out approximately the same horsepower, but the 2 stroke does it at 11,000rpm and the 4 stroke does it at 3,000 rpm. The same can usually be said for high and low kv electric motors. Assuming the same voltage, a high kv inrunner with a small diameter propeller would be perfect for a high speed model like a pylon racer, and a low kv motor with a large diameter prop will be better for power i.e. getting a sailplane to altitude, or 3D manoeuvres like prop hanging. Kv is determined by the number of winds or turns. This is the amount of times the copper wire has been wound around each stator pole. More winds = low kv, less winds = high kv.

kv has two main implications.

A high kv motor will spin faster than a low kv motor at the same voltage. This means you may choose to use a high kv motor if you are limited to a lower voltage battery pack. An example of this would be in 7 cell glider competition (7 NiMh or NiCd cells at 1.2 volts a cell = 8.4 volts). A lower kv motor could not produce enough rpm at 8.4 volts to be competitive, so a higher kv motor is used.

If you are not limited to a particular voltage, a lower kv motor can be used at higher rpm by using a higher voltage. Large outrunners with a kv of 200 to 300 are a good example of low kv high voltage motors. Make sure you consider the voltage limit for any motor you are considering.

Gearboxes (By wolw)

Choosing an inrunner and a gearbox isn't as complicated as it sounds, it is basically the same as choosing an outrunner but you add the downshift of the gearbox to the calculation.

When choosing an inrunner you usually have two sizes to choose from. I'll use Feigao for this example, Feigao have their motors listed on their site Feigao.com with a complete set of numbers on every motor and they even have suggestions for different set-ups. Feigao call their smaller diameter motors (27.6mm) "380" and their larger ones (36mm) "540". Both come in three different sizes, Small, Large and XLarge. These are copies of Hacker inrunners, "380" for the B40 size and "540" for the B50 size which makes it very easy to find an successful set-up to copy if you search the net. The ratios are the same for both Hacker and Feigao gearboxes 4.1:1 for "380" size and 6.7:1 for "540" size.

But what does that 6.7:1 mean ? To get the kv of the prop shaft with a gearbox, simply divide the kv of the motor by the ratio of the gearbox. If we take the FG540-07S as an example, it's a 5070kv motor but using a gearbox with a ratio of 6.7:1 you get a kv of 5070/6.7 = 757. This would fit a 3S setup for a hotliner perfectly. If you want to use it to it's max amp rating (93A) check that your Lipos are up to it (Check C Rating in this Guide). Personally I go a little easier on the Feigao motor/gearbox combinations when copying a Hacker set-up, as Hacker are are a higher quality product.

Peak Efficiency is a very good site where you can see how your intended setup will perform. Nothing is as precise as your own measurements, but it gives you some idea of what to expect. The graph isn't very easy to read but you'll get the hang of it if you read the "how do I read this chart" first.

Electronic Speed Controls (ESC’s)

There are two main types of ESC, for brushed or brushless motors. You cannot use a brushed ESC with a brushless motor or vice versa. Think of the features you will need like a brake and soft start. You will need a brake if you are using a folding prop and a soft start if you are using a gearbox and an on/off switch for a throttle. These features can often be found on Radio Controlled Sailplanes. The most important thing to consider when choosing an ESC is matching the ESC to your motor. It is good to use an ESC rated at a higher amperage than you intend running your motor at as an insurance against over stressing your ESC causing failure and potential damage to your model. Often you will see a burst rating for an ESC, meaning you can run the ESC at a maximum Amperage for a limited time, and exceeding this limit is asking for trouble. Most sensible aeromodelers like to have an esc capable of 10 to 20% more Amps than they plan to use depending on its quality. You will need a meter to measure the Amps and Volts being generated by your power system to ensure you are not stressing the battery, ESC or motor.

What is a bec? Bec is an acronym for battery eliminating circuit. This device provides power for the servos in your model. Many ESC’s have a bec that can only handle a certain number of servos at a given voltage. The higher the voltage you use the less servos you can use. Using too many servos from the bec in your ESC will cause overheating and failure of the bec. This will be catastrophic if your bec fails in flight so how can you safely run more servos with your ESC? External bec’s, or Ubec’s use power from your flight battery pack and are a cheap way of safely using more servos than the bec in your ESC can handle. A receiver battery pack is another way of supplying reliable power to your servos without using the bec in your esc.

Cut off voltage

Set the cut off voltage on your ESC to 3 volts per cell to ensure you don’t over discharge and damage your lipo pack.

Prop selection

The propeller is the component that puts a load on a power system. With the wrong prop you can damage your battery, ESC and motor. Think of the prop like the gears in a car. Some props are like first gear and the motor will have to work at high rpm to go slowly. If you have driven a 4X4 you will know that this gives you power to climb steep hills at low speed without stalling the engine. You could compare this to prop hanging a 3D model where power is more important than speed. On the other hand you might want to go fast. This will require a prop that is more like the top gear in a car. It doesn’t have the power to take off and climb a steep hill at low speed, but once up to speed it can maintain that speed comfortably. The numbers on a prop, say 10X4, give you the diameter and pitch. In this case you would have a prop with a diameter of 10 inches and a pitch of 4. A 10X4 prop will give you more thrust at a lower speed like the 4X4 analogy above. If you swapped it for a 10X7 prop you would have a higher top speed, but your take off run would be longer. The extra load on the motor would also draw a higher Amperage.

Pitch and Pitch Speed

Pitch is the distance (normally expressed in inches) that the propeller "cuts" through the air in a single rotation assuming no slippage. To achieve pitch, the propeller blades are angled to move air to create thrust. The angle of the blade determines its pitch. Propeller blades are aerofoils, just like the flying surfaces on our models. When they have a higher angle of attack they create more lift. In the case of propellers, a higher angle of attack (pitch) at a given rpm will create greater thrust.

Pitch speed is the speed at which the propeller pulls through the air. It is calculated by looking at the pitch of the propeller, and the number of revolutions it performs in a unit of time. Pitch speed does not consider slippage, drag and other forces that may affect the aircraft.

With a high wing loading you need a higher air speed to stay in the air. A higher pitch speed means lower thrust > longer take off > high landing speed. You can get both thrust and high air speed but it will be at a weight penalty as the power needed to get thrust for a short take off will not be in proportion to the power needed to stay airborne. Warbirds are an often examples of models with high power/high wingloading which are supposed to fly fast, and especially in glow to electric conversions you will need to take the wing loading into account. Hotliners and F5b models are one of the most extreme examples of high power/high wingloading. The more extreme examples have such a high pitch speed a catapult is needed to get them airborne because of the square (16x16) or over square (16x17) props they use in order to get extreme high speed/climbs. In a perfect world (with zero airframe drag and 100% prop efficiency) you can calculate the speed of your model from RPM x pitch)/1056 = your speed in mph. For example 10000rpm x 7" pitch /1056 =66mph or 105.6 km/h.

Pitch speed isn’t only about wing loading it's also about what you want to do with your model, as I wrote above about hotliners and F5b. With an already light model or of moderate weight you can determine the behaviour from the choice of prop > pitch speed. Without the need of changing anything (keeping the same amps) you can take a GWS Formosa II with a 10x5 from being a sporty low wing aerobatic trainer to a fast aerobatic plane with a 9x6. As a general rule 1" pitch relates to 1" of diameter, if you step up 1" in pitch you need to step down 1" in diameter to keep the same amp draw.With more normal kind of planes we usually use a prop with the proportion of 1:2 i.e. 10x5, 11x5.5, 12x6 and so on as it is most effective (from what I heard). A High wing trainer could very well use a more square prop like 9x7 instead of 11x5.5, it'll still have a high lift and once airborne you can throttle down, the higher pitch will give it airspeed and you'll get long flying times with low amps, perfect for photography or video.

The following is some extra information about prop selection kindly offered by brucea from RC Groups.

“As a rule of thumb, you want to have a static pitch speed within the 2.5 to 3 times the stall speed. So if your plane stalls at 15 mph in level flight you would like a static pitch speed between 37.5 to 45 mph.

For a particular motor, I know from testing that with a 12x6" propeller the motor is running at 7165 RPM. Each revolution pulls the plane forward 6". So my plane would be making 6" x 7165 RPM or 42,990 inches per minute. Dividing by 12" gives me 3,582.5 feet per minute. Multiplying my 60 minutes gives me 214,950 feet per hour. Dividing by 5280 feet gives me 40.7 miles per hour. The plane I has a calculated stall speed of 14 mph. 40.7 divided by 14 equals 2.9. This ratio falls within the desired 2.5 to 3 ratio of pitch speed to stall speed, which is good!

To select a motor you may have to work back-wards from prop diameter. The plane I have can take a 12" prop. I like to get the largest diameter prop that will fit.”

Broadly speaking, the "C" rating is a guide to how much current it is safe to draw from your battery. It's expressed in terms of the capacity or C. Beware that constant discharging of your lipo pack at its maximum C rating will almost definitely shorten its life. Depending on the quality of your pack, it is much wiser to keep your current draw to about 10 C with short bursts up to 20 C if you want your pack to last. The easier you are on your pack the longer it will last.

A 2200mAh 10C battery is rated to be discharge at up to 22A (10 x 2200mA/1000) and the same size 12C battery would be good for 26.4A (12 x 2200mA/1000).

The internal resistance in higher C rated packs is lower, meaning that the voltage drop found in higher C packs is not as pronounced giving higher voltage under load and slightly more power.

The motor draws 29 Amps from the 15C 30Amp discharge battery, and 31.5 Amps from the 20C 40 Amp battery. For this motor I am using two 2100 mAh 11.1v 15C batteries in parallel. This gives me twice the "C" rating or 60 Amps. This particular motor pulls 31 Amps with a 12x 6" prop and two 11.1V 15C 2100 mAh batteries.”Also beware that lipo manufacturers often put an overly optimistic C rating on their packs. Unless you see independent test results you trust for lipo packs, use them at about half the stated C rating and you should get many more cycles from them.

mAh

mAh is an acronym for Milliamp Hour, which is how much current a battery will discharge over a period of one hour. Higher numbers here reflect a long battery runtime and or higher storage capacity. For example a 2000 mAh pack will sustain a 2000 milliamp (2 amp) draw for one hour before dropping to a voltage level that is considered discharged. A 1700 will sustain a 1700 mAh (1.7 amp) draw for one hour. 1000 mAh is equal to a 1 Amp Hour (AH) rating.

Like the C rating, the mAh rating also determines the maximum current that can be drawn from a pack as can bee seen in the calculation in the C Rating section above. For example if you have three 11.1 Volt 10C packs, one rated at 1000 mAh, one rated at 1700 mAh and the other at 2000 mAh, we can determine that it is safe to draw the following amperage from these packs. Multiply the C rating by the mAh rating and divide by 1000 to convert milliamps to Amps:
10 X 1000 mA/1000 = 10 Amps
10 X 1700 mA/1000 = 17 Amps
10 X 2000 mA/1000 = 20 Amps

Credits

Thanks to the following for their help and contribution to this guide.

Peter (wolw at RCG) for his contribution to the sections on pitch speed and gearboxes.

Bruce (brucea at RCG) for his contribution to the sections on wing loading, prop selection and C rating.

Here is another cut at it that takes a slighty different slant but arrives at about the same conclusions. Note that you do not have to know this stuff in order to fly electric. RTFs have this all taken care of for you. Many ARFs recommend the right motors. However as you progress, as you look to upgrade or if you start designing your own planes, you will want to know the info provided by Chris F above, and what follows.

SIZING POWER SYSTEMS FOR ELECTRIC AIRPLANES
by Ed Anderson
aeajr on the forums
Revised 9/8/07

This may get a little technical but I will try to keep it as simple as I
can. I will draw parallels to cars and bicycles in many places as most
people can relate to these and know at least a little about how they work.
I will use round numbers where I can and will use some high level examples.
If you are an engineer you will see that I am taking some liberties here for
the sake of simplicity. I will go through the parts of the power system,
then, toward the end, I will show you how we tie these all together to come
up with a complete power system.

POWER = WATTS

I will be using the terms Volts, Amps and Watts throughout this discussion.
Let me define them.

Volts = the pressure at which the electric energy is being delivered - like
pounds per square inch or PSI in a fuel system or water from a garden hose.
Volts is about pressure, it says nothing about flow. You will see volts
abbreviated as V.

Amps = the quantity or flow of electricity being delivered, like gallons per
minute in a fuel system or that same garden hose. Amps is about flow, it
says nothing about pressure. You will see amps abbreviated as A.

Watts = V X A. This is a measure of the energy or power being delivered.
This is how we measure the ability of that electricity to do work, in our
case the work of turning a propeller to move our airplane through the air.
Watts is about both pressure and flow. This serves the same purpose as
the horsepower rating of your car's engine. In fact 746 watts = 1
horsepower. So if you had an electric car, the strength of its motor could
be reported in either watts or horsepower. You will see watts abbreviated as
W.

Whether brushed or brushless, the motor's job is to convert electricity into
mechanical motion to turn the propeller to move air. Efficiency is how we
measure how much of the power, the watts, that our battery delivers to the
motor is actually turned into useful work and how much is wasted as heat.
A higher efficiency motor delivers more energy to the prop, and wastes
less.

A typical brushed motor, say a speed 400, is only about 40-50% efficient.
Only about half the watts delivered to the motor actually end up as useful
work turning the propeller. The rest is wasted. Motors that have a "speed"
designation, like speed 400, are brushed motors. There are other names for
brushed motors but the "speed" term is a common one. They are inexpensive
and they work. For example, you can buy a speed 400 motor and electronic
speed control, ESC, for $30. A comparable brushless motor/ESC combination
would
typically cost 2 to 4 times that much.

Brushless motors tend to be more efficient. They typically deliver 70-90%
of that input power to the propeller, Thus you get better performance per
watt with brushless motors. Seen a different way, if you use a brushless
motor, then, for the same flying performance you will use less energy which
means your battery will last longer. Or you can use a similar size and
weight brushless motor.battery combo to get comparable performance
because the motor turns more of the watts from the battery into useful work
of turning the propeller.

As with many decisions we make, this is a cost benefit decision. Am I
willing to pay more to get more? That is up to you.

THE BATTERY IS MORE THAN JUST THE FUEL TANK

Think of the battery as the fuel tank plus the fuel pump and a supercharger
all rolled into one. It feeds/pushes energy to the motor. So you have to
look at the battery and the motor as one unit when you are sizing power
systems for electric planes. In many cases we start with the battery when
we size our systems because the motor can't deliver the power to the prop if
the battery can't deliver the power to the motor.

The higher the voltage rating of the battery, the higher the pressure, like
a supercharger on a car engine. More pressure delivers more air/fuel
mixture to the engine which allows the engine to produce more power to turn
the wheels of the car.

Higher voltage pushes more electricity into the motor to produce more power,
IF AND ONLY IF, the battery has the ability to deliver more electricity.
Again using the car analogy, if you put a big motor in a car and put a tiny
fuel line and a weak fuel pump, the motor will never develop full power. In
fact the motor might starve and stall once you got past idle. Such is the
same with batteries. We need voltage, we need capacity, but we also need to
know how many amps the battery is capable of delivering at peak.

If we compare an 8 cell AAA battery pack to an 8 cell C battery pack we get
9.6 V for both packs. However the AAA pack may only be able to deliver 6
amps. After that the cells will heat up and either be damaged or the
voltage will start to drop fast. The C pack, also 9.6 V, might be able to
deliver 60 amps without damage. So we have to size not only by voltage, but
by the ability to deliver amps to the motor. Again, think of the fuel line
and the fuel pump as your image of what I am trying to explain. If the
motor needs 12 ounces per minute to run but the fuel line can only deliver
8, the engine will starve and die.

Using our electric motors, a given motor may take 10 amps ( the quantity of
electricity flowing ) at 8.4 volts ( the pressure at which the electricity
is being delivered) to spin a certain propeller. We would say that the
battery is delivering, or that the motor is drawing 84 watts, i.e.: 8.4V x
10A. If you bump up the voltage to 9.6 volts, the battery can ram in more
amps into the motor, more energy to the motor, which will produce more power
to the propeller. In this example, if we move from an 8.4V battery pack to a
9.6V battery pack the motor may now take 12 amps. This will typically spin
the motor faster with any given propeller or allow it to turn a larger
propeller at the same speed.

However, if you bump up the pressure too much, you can break something.
Putting a big supercharger on an engine that is not designed for it will
break parts of the engine. Too much voltage can over power your electric
motor and damage it. So there is a balance that has to be struck.
Different motors can take different amounts of power, watts, volts X amps,
without damage. For example, a speed 400 motor might be fine taking 10 amps
at 9.6 volts or 96 watts. However bump it up to 12 volts and ram 15 amps
down its throat and you will likely burn it out.

Our goal is a balanced power system. If you match the right battery with
the right motor, you get good performance without damage to the motor. In
many cases airplane designers will design planes around a specific
motor/battery combination so that they match the size and weight of the
plane to the power system for good
performance.

PROPELLERS

Propellers are sized by diameter and pitch.

The diameter of the propeller determines the volume of air the propeller
will move, producing thrust, or pushing force. Roughly speaking the
diameter of the propeller will have the biggest impact on the size and
weight of the plane that we can fly. Larger, heavier planes will typically
fly better with larger diameter propellers.

Pitch refers to the angle of the propeller blade and refers to the distance
the propeller would move forward if there were no slippage in the air. So a
7 inch pitch propeller would move forward 7 inches per rotation, if there
were no slippage in the air. If we combine pitch with the rotational speed
of the propeller we can calculate the pitch "speed" of the propeller. So,
at 10000 revolutions per minute, that prop would move forward
70,000 inches per minute. If we do the math, that comes out to a little
over 66 miles per hour.

By changing the diameter and the pitch of the propeller we can have a
similar effect to changing the gears in your car or a bicycle. It will be
harder for your motor to turn a 9X7 propeller than an 8X7 propeller. And
it would be harder to turn a 9X7 propeller than a 9X6 propeller. The
larger or steeper pitched propellers will require more energy, more watts,
more horsepower, to turn them. Therefore we need to balance the diameter
and pitch with the power or wattage of the motor/battery system.
Fortunately we don't actually have to do this as motor manufacturers will
often publish suggested
propellers to use with a given motor/battery combination. We can use these
as our starting point. If we want we can try different propellers that are
near these specifications to see how they work with our airplane.

GEARBOXES

While unusual on glow or gas planes, gearboxes are common on electric
planes. Their primary function is similar to the transmission on a car. The
greater the gear ratio, the higher the numerical value, the slower the
propeller will turn but the larger the propeller we can turn. So you can
use a gearbox to help provide more thrust so you can fly larger planes with
a given motor. However you will be turning the propeller slower so the
plane will not go as fast.

With direct drive, that is when the propeller is directly attached to the
motor shaft, we are running in high gear ( no gear reduction). Like pulling
your car away from the light in high gear. Assuming the motor doesn't stall,
acceleration will be slow, but over time you will hit a high top end!
Typically direct drive propellers on a given motor will have a smaller
diameter.

With the geared motor, it would be like pulling away from the green light in
first gear - tons of low end power and lots of acceleration, but your top
speed is reduced.

So, by matching up the right gear ratios made up of the propeller and,
optionally, a gearbox we can adjust the kind of performance we can get out
of a given battery/motor combination. How this is done is beyond the scope
of this article.

NOW WE CAN START TO MATCH UP THE PIECES!

The simplest approach I have seen to figuring power systems in electrics is
input watts per pound of "all up" airplane weight. The following guidelines
were developed before brushless motors were common but it seems to hold
pretty well so we will use it regardless of what kind of motor is being
used.

50 watts per pound = Casual/scale flying

75 watts per pound = Sport flying and sport aerobatics

100 watts per pound = aggressive aerobatics and perhaps mild 3D

150 watts per pound = all out performance.

Remember that Watts = Volts X Amps. This is a power measurement.
In case you were wondering, 746 watts equals 1 horsepower.

AN EXAMPLE!

This should be fun. Let's see where these formulas take us! We will use a
24 ounce, 1.5 pound plane as our example. If we want basic flight you will
need 50 watts per pound or about 75 watts input to your motor for this 1.5
pound plane. That is, 50 watts per pound X 1.5 pounds = 75 watts needed
for basic flying performance. If you want a little more spirited plane, we
could use 75 watts X 1.5 pounds which is about 112.5 watts.

Lets use 100 watts as the total target, just to be simple, shall we? I am
going to use a lot of round numbers here. I hope you can follow.

The Battery:

If we use an 8 cell NiMh battery pack at 9.6 V it will have to deliver 10.4
amps to hit our 100 watts input target ( 100/9.6 = 10.41amps) If my
battery pack cells are NiMh cells that are rated at 10C then I need an 8
cell pack rated at 1100 mah to be able to deliver 11 amps. Sounds about
right.

Now I select a motor that can handle 100 watts or about 10.4 amps at 9.6
Volts. From experience we know this could be a speed 400, a speed 480 or
some kind of a brushless motor.

I see that if I use a direct drive speed 400 with a 5X4.3 prop at 9.6V then
the motor will draw about 12.4 amps or about 119 watts. This would be a
good candidate motor/prop for the plane using a 9.6V pack that can put out
12.4 or more amps. This would be a set-up for a fast plane as that motor
will spin that small prop very fast.

However maybe I don't want such a fast plane but one with a really good
climb and lots of low end pull to help out a new pilot who is in training or
to do more low speed aerobatics

I can also use a speed 400 with a 2.38 gearbox and run it at 9.6V spinning a
9X7 prop and run at about 12.8 amps for 120 watts.http://www.gwsus.com/english/product/powersystem/eps400c.htm
The larger prop will give this plane a strong climb, but since the prop
speed has been reduced by 2.38 times, it won't be as fast. Spinning a
bigger prop gives me more thrust but a lower top speed typically. This is a
common strategy for 3D planes.

Back to battery packs and motors

So if I shop for a 9.6V pack to be able to handle about 15-20 amps, I should
do just fine and not over stress the batteries. In NiMh that would probably
be a 2/3 or 4/5 A pack of about 1000 -1300 mah capacity. Some examples here:http://www.cheapbatterypacks.com/mai...ells&chem=NIMH

We view the battery and motor as a linked unit with a target power profile,
in this case about 100 watts. We use the prop and gearbox, if any, to
produce the manner in which we want to deliver that power to the air to
pull/push the plane.

If this is a pusher, I may not have clearance to spin that big prop so I
may have to go for the smaller but faster prop combo.

If this is a puller, then I can choose my prop by ground clearance or some
other criteria and match a gear box to it.

See, that was easy, right? ( well sorta but ....)

But we are not done! Oh no!

I could try to do it with a 2 cell lithium pack rated 7.4V. To get 100 watts
I now need a pack that can deliver 13.5 amps and a motor/prop combination
that will draw that much. So if I have 10 C rated lithiums, then the pack
better be at least 1350 mah. Probably use a 1500 mah pack to be safe.

Well, when I look at the chart for the geared speed 400 I see that,
regardless of prop, at 7.4V I am not going to have enough voltage (
pressure) to push 13 amps into this motor. So the 2 cell lithium won't meet
my performance goal of 100 watts+ per pound using this gear box.

We see that the best I can get this speed 400 to do is a total of 70 watts
at 7.2V ( close enough ) so I can't hit my power goals using a speed 400 at
this voltage. but 70 watts would be about 48 watts per pound so I could have
a flyable plane, but not an aerobatic plane using this two cell pack.

REALITY CHECK!

Now, in fact that is NOT how I would do this. I would decide on the watt
target, go to the chart, find a combo that meets my goals, then select a
battery that will meet the demand and see if my weight comes up at the
target I set. A little tuning and I come up with a workable combo.

I often use the MaxxProd combos for reference. If you read the details on each
package they have wonderful information. And, the fact is that I generally go
with brushelss motors these days. Costs are reasonable and their higher
efficency
gives me more performance and longer flight times.http://www.maxxprod.com/mpi/mpi-264.html

Following the example above, the combo 10 on that page would be an excellent
fit for my 1.5 pound plane for sport flying.

The Combo 049 might be a good fit for a slow flyer. Either way the package
has all I need.

If I wanted the plane to have all out performance, the 15A or 19A package would
be my pick. Note that these would need either higher voltage or higher amperage
battery packs. The flyers/PDF for the packages make recommendations.

For those who like to be even more analytical about it, there are packages
like MotoCalc that will allow me to play with all sorts of combinations and
make suggestions on what I should use. There is a link for MotoCalc below.

SUMMARY

So, in these few paragraphs you have taken in a basic knowledge of how electric
power systems are sized, the factors that are considered an how to predict
the outcome. Simple, right?

Of course there is a lot more to know and time and experience will teach
you plenty, but with this basic understanding you are better prepared to
begin playing with the power systems you put in your planes.

MotoCalc
This program will tell you everything you need to know: Amps, Volts, Watts, RPM,
Thrust, Rate of Climb, and much more! It is a popular tool for predicting
the proper motor, prop, battery pack for electric planes.http://www.motocalc.com/

This club has some interesting links on their home page that may be helpful
in planning props and power systems.http://www.srcmc.co.uk/

Drive Calculator Version 2.21
Based around a Microsoft Excel spreadsheet This includes a propeller thrust
and power database, in a similar form to MotorXL, a motor database, and
tools for predicting home made motor performance. With these tools it is
possible to predict the performance of motor and prop combinations - even
with custom motors!http://www.badcock.net/motorcalc/

Examples are extremely useful, because even if you don't know what the heck it all means, you can just use an example, maybe changing a few numbers, and have your setup. Thats probably what I'll be doing!

The diameter of the propeller determines the volume of air the propeller
will move, producing thrust, or pushing force. Roughly speaking the
diameter of the propeller will have the biggest impact on the size and
weight of the plane that we can fly. Larger, heavier planes will typically
fly better with larger diameter propellers.

Pitch refers to the angle of the propeller blade and refers to the distance
the propeller would move forward if there were no slippage in the air. So a
7 inch pitch propeller would move forward 7 inches per rotation, if there
were no slippage in the air. If we combine pitch with the rotational speed
of the propeller we can calculate the pitch "speed" of the propeller. So,
at 10000 revolutions per minute, that prop would move forward
70,000 inches per minute. If we do the math, that comes out to a little
over 66 miles per hour.

By changing the diameter and the pitch of the propeller we can have a
similar effect to changing the gears in your car or a bicycle. It will be
harder for your motor to turn a 9X7 propeller than an 8X7 propeller. And
it would be harder to turn a 9X7 propeller than a 9X6 propeller. The
larger or steeper pitched propellers will require more energy, more watts,
more horsepower, to turn them. Therefore we need to balance the diameter
and pitch with the power or wattage of the motor/battery system.
Fortunately we don't actually have to do this as motor manufacturers will
often publish suggested
propellers to use with a given motor/battery combination. We can use these
as our starting point. If we want we can try different propellers that are
near these specifications to see how they work with our airplane.

Ok, let me see if I got this straight. We'll use my Easy Star as an example. Currently, it's equipped with the stock prop, which is a 5"X4.4", Permax 400 brushed motor and a 7-cell NiMH pack (8.4V).

Originally Posted by AEAJR

However maybe I don't want such a fast plane but one with a really good climb and lots of low end pull to help out a new pilot who is in training or to do more low speed aerobatics

I can also use a speed 400 with a 2.38 gearbox and run it at 9.6V spinning a
9X7 prop and run at about 12.8 amps for 120 watts.http://www.gwsus.com/english/product...em/eps400c.htm
The larger prop will give this plane a strong climb, but since the prop
speed has been reduced by 2.38 times, it won't be as fast. Spinning a
bigger prop gives me more thrust but a lower top speed typically. This is a
common strategy for 3D planes.

Now, if I were to use Chris' 1" for 1" formula; "As a general rule 1" pitch relates to 1" of diameter, if you step up 1" in (diameter) you need to step down 1" in (pitch) to keep the same amp draw." instead of your more exaggerated example, then by simply swapping the current 5"x4.4" prop with a 6"x 3.4" prop, I could still use the same motor and electronics, as there would be no increase in power requirements (amperage draw). And while the plane would have a slower top speed, it would be able to climb better, i.e. at a steeper angle. Correct?

Or have I missed something?

OK, I got my "drone licence"... When does the season start and what Ammo can I use?

Thanks Ed. People have put 6" props on the EZ* using an prop extender. Measuring the clearence on mine I've determined the largest diameter the plane will allow safely (stock, no extention) without cutting a channel in the Fuse is 5 3/4".

OK, I got my "drone licence"... When does the season start and what Ammo can I use?

This guide was started by me but is being added to by members of various forums. If you have an addition for this guide send me a PM and I will make any necessary changes or additions. Any input to the guide will be credited.

Like the C rating, the mAh rating also determines the maximum current that can be drawn from a pack as can bee seen in the calculation in the C Rating section above. For example if you have three 11.1 Volt 10C packs, one rated at 1000 mAh, one rated at 1700 mAh and the other at 2000 mAh, we can determine that it is safe to draw the following amperage from these packs. Multiply the C rating by the mAh rating and divide by 1000 to convert milliamps to Amps:
10 X 1000 mA/1000 = 10 Amps
10 X 1700 mA/1000 = 17 Amps
10 X 2000 mA/1000 = 20 Amps

Credits

Thanks to the following for their help and contribution to this guide.

Peter (wolw at RCG) for his contribution to the sections on pitch speed and gearboxes.

Bruce (brucea at RCG) for his contribution to the sections on wing loading, prop selection and C rating.

I get questions about gearboxes from time to time. They are covered above, but if you are still a little unclear on when and why a gearbox is used, this may help.

We are going to discuss why we would consider adding a gearbox to a brushed
electric motor.

CHANGING PROPS IS LIKE CHANGING GEARS

I am going to get real loose with the words "gear ratio" for a moment, but try
to follow me. Think of gear and gear ratio as the way we adjust the load on the
motor. I can adjust the "gear ratio" on my motor/propeller set-up in one of two
ways:

1) change the propeller
2) add a gear box and change the propeller

The goal is to get the motor spinning, at full power, at its optimum watt range
so that we do not over burden it, but so that we get the power to the propeller
efficiently. We are trying to get the best balance between pitch speed, thrust
and current draw.

If I increase the diameter of the propeller while holding the pitch constant I
put a greater load on the motor. A 10X6 prop puts a greater load on the motor
than a 9X6 prop. It will cause the motor to draw more power, more amps. At the
same time, it may load it enough that it causes it to slow down. Its peak RPM
may will be less. This is similar to changing gear ratios on your bicycle.
You can feel the effect in your legs.

If I deepen the pitch on the propeller while holding the diameter constant, I
also increase the load on the motor. A 9X6 going to a 9X7 going to a 9X8. In
this case I am increasing the "pitch speed". Again, this is similar to changing
the gear ratio. As I go to a deeper pitch the current draw will increase, the
watts increase and we may again load the motor enough to decrease its top rpms.

If I go too wide, or too deep, I can overload the motor and burn it out.

So, on a direct drive set-up, no gearbox, I tune my propeller between pitch and
diameter to get the motor to the power range I want. Again, this is EXACTLY the
same as changing gear ratios, in practical application.

To some extent I can trade pitch for diameter and vice versa. So you will see
motors listed as accepting a range of propellers. Typically as diameter goes
up, pitch goes down.

9X7
10X6
11X5

For this sample motor, each of these props will probably produce a similar watt
output but they do it with different results.

The wider prop will provide more thrust but the lower pitch will produce less
speed. So I can tune for the application. Sailplanes typically want more
thrust for steeper climb but are not as concerned about speed. Pylon racers
are less concerned about climb or acceleration as they are about top speed.
Hopefully you get the idea. I am tuning the "gear ratio" by changing the prop.

If you are not with me up till now, then ask because what comes next depends on
your understanding what is above.

ADD A GEARBOX

Now, suppose I have a given motor, say a brushed 550, and my prop choices don't
give me the thrust I want to take my 2 meter sailplane up at a steep enough
angle to make me happy. It takes too long to get to soaring height. Or,
suppose I want to fly a larger, heavier plane with the motor I have. My prop
choices don't give me enough thrust to handle the heavier plane. What do I do?

I can put in a gear box. The gearbox will have two effects. It will reduce the
top speed to the prop, but it will increase the torque available to turn the propeller.
This allows me to go to a wider propeller but my top speed will be reduced. Now
I can get an steeper climb, or perhaps I can fly a larger or heavier plane. I
am going to stay with the sailplane for the rest of the discussion, but it
applies equally to any kind of aircraft. We are talking gear ratios.

Again, using the bicycle example, you shift to a lower gear to go up the hill.
You can
get up the hill in first but if you were to go to third you might not have
enough power in your legs to turn the pedals. So you tune the gear ratio to
match the available power.

A typical prop on a 550 motor in a sailplane, like a Goldberg Electra would be
an 8X4 prop. That is the widest prop, the highest thrust prop that this motor
can comfortably turn and provide enough speed to fly the glider. The motor will
likely pull about 18 amps on an 8.4V pack. It will fly the plane but the climb
angle might only be 25 degrees. So it might take me 2 minutes to fly up the
height I want to reach. This plane isn't really made for speed, so going to a
7X6 prop, trying to get more speed, won't help.

But if I put a gear box on, say a 3:1 ratio, I can go to an 11X8 or a 12X7 prop.
Now I get a lot more thrust and the plane will climb at a 50 degree angle. Now
I get to height in less than a minute and the motor might only be pulling 16
amps. I climb in less time AND I may be drawing fewer watts to do it.

That is why we go to a gear box. Usually it is to allow us to swing a wider
prop at a slower speed in order to get more thrust at the sacrifice of speed.

I hope that helps.

WHAT ABOUT BRUSHLESS INRUNNER VS OUTRUNNER?

Because we have two motor types in the brushless world we add flexibility and
complexity. More choices means more to decide.

The gearbox discussion with a brushless inrunner is exactly the same as for the
brushed motor above, so I won't repeat it.

However if we look at outrunners vs. inrunners we see that outrunners tend to
spin slower/volt with more torque. this has a similar effect to having a
gearbox on an inrunner. So how do you decide?

Some people don't like gearboxes. It is another thing to maintain and another
thing to break. Also gearboxes tend to make noise and some people don't like
that. however there is nothing spinning around inside the plane with a gearbox.
So you can mount the motor/gearbox without regard to clearance as long as you
have adequate air flow. You can just clamp a gearbox/inrunner to the frame of
the plane and you are done. I have seen motor/gearboxes left loose in the nose
of the plane. The Multiplex Easy Glider is set-up this way. No mount at all,
it just sits there.

Outrunners need space. You have a spinning can that must be protected from
contacting another surface, lose parts, wires, etc. Grass, string, stuff can
get caught on that spinning can. In some cases this could be a problem, so a
gearbox might be preferred.

I have read that brushless inrunners are typically more efficient than
outrunners. Even with the gearbox losses I have read that inrunners are still
more efficient at turning those bigger props. So, if that is true, and if that
matters, it could shape your decisions.

Glad you find the discusison helpful. The answer to your question is typically yes. For a give amount of available power, measured in watts, if you increase the pitch, you will have to decrease the diamater to maintain the same power input/output.

Now, if you have more power avaiable than what the current prop is using, in increase in pitch on the same diamater prop will likely produce higher speed at similar or higher thrust.

Example:

Motor/battery combo is rated for 200 watts.

Currently you have a 9X7 prop on and when you mesure with the watt meter you are producing 160 watts. Your battery is holding at 10V are holding at 16 amps = 160 watts.

So you are not at max yet. It only takes 160 watts to spin that prop. So, if you go to 9x8 prop, you might pull 180 watts. Again let's assume your battery still holds at 10V but now you are drawing 18 amps.

Your RPM should be close to the same but you are moving through the air faster.

This is, of course theroy. With the deeper prop, the voltage may sag to 9.6V so your RPM will be down, based on the Kv rating of the motor. This may result in reduced thrust.

This is all concept. Actual tests may show slightly different results.

Anyone have more experience with this? Or anyone with moto calc and want to model this?

Hello folks..ok so im going to build a plane over the winter months to come this year . Im a newbie to this and i have successful flights with my Cub and my parkzone Spitfire and my girlfriend is getting me a Spektrum DX7 for christmas so i want to build a plane but i need to gather info and what i need to purchase to fully get my plane running as i usually buy RTF models so i never have to build anything except put the wings on and charge the battery and then im good to go so i definitely need alot of info on what kind of motors are good and how to match your ESC with the motor and all that jazz!!. . Thanks

Hello folks..ok so im going to build a plane over the winter months to come this year . Im a newbie to this and i have successful flights with my Cub and my parkzone Spitfire and my girlfriend is getting me a Spektrum DX7 for christmas so i want to build a plane but i need to gather info and what i need to purchase to fully get my plane running as i usually buy RTF models so i never have to build anything except put the wings on and charge the battery and then im good to go so i definitely need alot of info on what kind of motors are good and how to match your ESC with the motor and all that jazz!!. . Thanks

You'll need to supply us with a bit more information in order to help you out. What type of plane are you building? Building from a kit or scratch? If from a kit which one (they usually have recomendations for motor or at the minimum how many watts). How big of a plane are you building?

As far as matching a motor with an ESC you want to make sure that the ESC is capable of handling the maximum amount of Amps that your motor will draw. You should be able to find that information from the motor distributor or manufacturer. They usually have a max efficiency amp rating and a maximum amp rating. You always over compensate with the ESC to be on the safe side. If your motor says max 25amps get a 30 or 36 amp ESC.

Hello, I,m interested in the Hobby City C20-2050kv brushless outrunner motor, using the Zippy 13002S20C,1300 MA, ZSIP 20C Lipo battery, and a SUP20A SuperSimple 20Amp ESC. Is this a combination that will work for a 20-26 ounce glider? Can I use a 6/3 folding prop with a 26 to 28mm spinner?